The present invention relates generally to a method of repairing welded joints in aluminum sheet materials and, more specifically, to a method of repairing undersized aluminum resistance spot welds.
Resistance spot welding of aluminum alloy sheets is often implemented in the production of automotive body parts, such as deck lid panels, body frame members, or the like. These aluminum sheet alloys often have compositions in the Aluminum Alloy 5xxx and 6xxx series. Several spot welds in the assembly of automotive body members and closure panels are common. In the joining of aluminum sheets to make these parts, the sheets are held in place and aligned while opposing copper electrodes are pressed against opposite sides at the weld location. Electrical current is passed from one electrode through the underlying sheets to the other electrode. Heat generated by resistance to the flow of electric current through the work pieces melts some of the metal at their interfaces. Current flow is stopped and the sheets are held together until the molten nugget cools and solidifies to bond the sheets at the weld spot. The electrodes are then retracted.
Often the cooperating electrodes have sheet contacting tips that are flat or dome shaped for applying local pressure to the sheets and for good electrical contact. The electrodes are often cooled for their own protection and to help dissipate heat from the welding region when current flow has been stopped. A spot weld of suitable size and quality depends significantly on the proper alignment and pressure of the electrode tips and the duration and amount of current flow. Weld quality also depends upon the surface condition of the electrode face. Usually the size of the weld nugget corresponds to the contact area of the electrode faces. It is not unusual to make several spot welds in the assembly of automotive body members and closure panels.
A suitable weld nugget is generally round (like the weld electrode tip) in plan view and oval in vertical cross section. Spot welding operations are often conducted in a highly repetitive operation and sometimes problems or variations occur in the control or execution of the process and undersized weld nuggets are produced. Misapplication of welding current or unsuitable positioning of an electrode tip on the surface of the aluminum sheet can produce undersized, weak welds. Usually undersized welds must be repaired and thus, a practical repair process for spot welds in aluminum sheet materials is urgently needed.
For aluminum resistance spot welding, unlike resistance spot welding of stainless steel, the welding heat is primarily created due to the electrical resistance of aluminum oxide present in the aluminum alloy material. But the oxide is dispersed or depleted once the first weld has been made. Thus, it is ineffective to try to re-weld an existing weld nugget using the same resistance spot welding practice and equipment because the previously used current path is now too conductive to melt additional metal. Similarly, attempting to repair a weld by adding additional spot welds around the weak spot is not practical and if its new welds are too close to the original one, current shunting will often create new undersized welds.
Thus, it is an object of the present invention to provide a practical and efficient method of repairing undersized aluminum spot welds thus increasing the size and durability of the existing weld.
The present invention provides a method of repairing an undersized aluminum resistance spot weld formed between two aluminum alloy sheets. In accordance with the invention, a supplemental weld is formed around the periphery of the undersized nugget to form a composite weld body between the sheets having suitable size and strength. The original undersized weld nugget is generally round in plan view. In this case, the supplemental weld is a donut shaped or toroidal shaped nugget that surrounds the original nugget and, preferably, is fused to it to make a more or less unitary weld nugget of desired size.
The practice of the method requires electrodes sized and shaped to form the supplemental weld nugget. For example, a pair of copper welding electrodes with donut shaped tips is placed, one on each of the outer surfaces of the defectively welded aluminum alloy sheets. The tips of the opposing electrodes are positioned on the sheet surfaces directly over the electrically resistive aluminum alloy surrounding the undersized weld nugget.
The copper welding electrodes are pressed against the aluminum sheets at the welding region for good electrical contact. Once secured, an electric weld current is supplied that flows between the electrodes, through the aluminum sheets, to the sheet-to-sheet contact interface area located at the welding region. Thermal energy is created at contact surfaces of the aluminum sheets caused by resistance of the aluminum oxide and aluminum alloy to the electric current. A portion of the aluminum alloy sheets melts between the contact points of the electrodes, thus creating a new weld nugget around the undersized one. This new weld nugget is of a donut shape, in conformance with the size and shape of the electrode tips, and it surrounds the periphery of the existing undersized spot weld.
The welding current is suitably alternating current (AC), typically and suitably 60 Hz, or direct current (DC). Often DC for welding is produced by rectification of 60 cycle AC and such DC has the cyclic nature of the AC. After a suitable duration of current flow, e.g., 5 to 8 cycles of AC (60 Hz) or cyclic DC, current flow is stopped and the molten supplemental weld nugget is allowed to fuse onto and around the existing undersized weld. While maintaining electrode pressure on the aluminum alloy sheets, the composite weld nugget is allowed to cool and solidify. As a result, a repaired and properly sized aluminum spot weld is thus formed.
This and other aspects of the present invention will become apparent from a detailed description of specific embodiments that follow.